Wheelchair harness.

I was wondering what type of straps or harness systems guys in particular have in place when using their wheelchair. I don't use any but came headfirst out of my chair this morning when my casters hit a ridge at a garden centre I was attending and I had not spotted. Not good feeling very shaky. Been looking at the various systems available and am leaning towards a chest cross over butterfly type. Reasons I don't want a lap strap is I have Foley catheter plus spinal cord stimulater implant in my right side just where the strap could cause a problem. My worries are getting it fitted on my wheelchair which is a simple no frills titanium tubular cyclone chair. I am due a new chair from my wheelchair services in February and have called them to say I want re assessment asap but it will be February at the earliest. There is a system available for £54.00 on the net looks ok but like I said how do I know it would fit. Just wondering any advice would be great.